There is known a technique for reducing an impact energy in the event of collision of a vehicle, with buckling of an intermediate portion of a propeller shaft of the vehicle, namely deformation of the intermediate portion in a direction of the collision. The intermediate portion has sections having respective different diameters. JP-2006-175938A discloses an example of this type of vehicular propeller shaft. This vehicular propeller shaft is configured such that a portion of the propeller shaft undergoes buckling deformation in a running direction of the vehicle, that is, in the axial direction of the propeller shaft, upon application of the impact energy to the vehicle in the rearward direction. The propeller shaft thus configured to absorb the impact energy generated in the event of collision of the vehicle permits reduction of an impact force to be applied to the vehicle upon its collision.
The vehicular propeller shaft is required to be reduced in weight, for improvement of fuel economy of the vehicle. For reducing the weight of the vehicle, it is effective to form the propeller shaft by using a material having a high degree of strength, for thereby reducing a required wall thickness of the propeller shaft. To this end, it is effective to increase the strength, that is, a tensile strength of a steel pipe or tube which is generally used for the vehicular propeller shaft. Where the material having a high degree of tensile strength, for example, a high tensile-strength steel tube is used for the propeller shaft, however, this material has a low degree of workability, and difficulty to form the propeller shaft having a complicated geometrical configuration for effective absorption of the impact energy in the event of collision of the vehicle.